The present invention relates to a surface reformation method of a high polymer material and, more particularly, to a surface reformation method of a high polymer material which is preferable to be used in order to improve wettability of a liquid used for temporarily immobilizing electronic parts on an electronic circuit to board the surface of which is covered by a high polymer material layer. The invention also relates to improvement of wettability of a liquid used for temporary immobilization of electronic parts when the electronic parts are soldered on an electronic circuit board according to a fluxless reflow soldering.
As a conventional technique regarding a surface reformation method for wettability improvement of a high polymer material, for example, methods using an O2 asher, Ar sputter, or these like are known. In the methods, a process is performed by setting a circuit board on which a high polymer material layer to be processed is coated in a vacuum vessel, and the surface reformation is executed for the entire surface of the high polymer material on the substrate.
According to the conventional technique using, for example, the O2 asher or Ar sputtering mentioned above, the surface of the substrate can be reformed only in an atmosphere of vacuum, and there is a problem such that a large-scaled apparatus including vacuum equipment, vacuum vessel, and the like is necessary. According to the conventional technique, since the surface reformation is executed for the entire surface of the high polymer material on the substrate, an area which is not necessary to be processed other than the area on which the electronic parts are temporarily immobilized on the electronic circuit board or the like is also reformed. Consequently, the liquid used for the temporary immobilization of the electronic parts wets the portion which is unnecessary for the temporary immobilization of the parts, so that there is a problem such that a large amount of liquid is wasted. As mentioned above, when the entire surface of the high polymer material on the substrate is reformed and the liquid for temporary immobilization wets even the unnecessary portion, even if the parts are temporarily immobilized, the parts are moved from positions of the temporary immobilization at the time of a reflow soldering, so that the object of the temporary immobilization cannot be achieved. Particularly, when the size of the substrate is equal to 50 mmxc3x9750 mm or larger, since the liquid for temporary immobilization tends to gather in the center and the temporarily immobilized parts easily move, it is not preferable. Hitherto, a flux for soldering has been commonly used in the temporary immobilization of the parts in the reflow soldering and the parts are not moved from the positions of the temporary immobilization. In case of performing the reflow soldering without using the flux, however, when the liquid for temporary immobilization wets the unnecessary portion, the object of the temporary immobilization is not achieved as mentioned above. It is, therefore, conventionally difficult to solder the electronic parts at predetermined positions on the electronic circuit board by performing a fluxless reflow soldering.
It is an object of the present invention to provide a surface reformation method of a high polymer material which solves the problems of the conventional technique and which can perform a surface reformation of a substrate in which wettability for a liquid employed for temporary immobilization is improved with respect to only an area necessary for temporary immobilization of electronic parts without needing vacuum equipment, vacuum vessel, and the like. The surface reformation here denotes the reformation of the properties of the surface for improving the wettability of the liquid for temporary immobilization.
The above-mentioned object of the present invention is achieved by irradiating the high polymer material surface with light energy, only at the area to be treated (that is, an area to be wetted by the liquid for temporary immobilization.
According to the present invention, by applying the light energy to only the area which needs the surface reformation of the present high polymer material, the surface of the only portion which is irradiated with the light energy can be reformed and the wettability for the liquid can be improved only in the necessary area. Consequently, the liquid for temporarily immobilizing the electronic parts and the like can make only the processed area wet, the processed area can be wetted by a minimum amount of the liquid, and the electronic parts can be certainly temporarily immobilized at predetermined positions on the surface of the high polymer material.
As a base material of a substrate on which the high polymer material layer is provided, any substrate used for an integrated circuit such as, for example, a printed board, glass plate, or ceramic plate can be used.
A polyimide system resin is used for the high polymer material layer on which the electronic parts are mounted. As the polyimide system resin, polyimide isoindroquinazolindion (PIQ; trade name of Hitachi Chemical Co., Ltd.) and polymethylmethacrylate (PMMA) can be usually employed. In addition, OFPR (trade name of Tokyo ohka Kogyo Co., Ltd.), and B20 (trade name of Hitachi Chemical Co., Ltd.), and are also suitable.
A wavelength of the light energy which is thrown onto the surface of the high polymer material lies in a range from 100 nm to 600 nm. When the wavelength of the irradiation light is too short, the irradiation light is transmitted to the inside of the high polymer material. When the wavelength is too long, abrasion of these high polymer material surface is insufficient. Both of the cases are not preferable.
An energy density of the irradiation light is set to be larger than 0.03 J/cm2 and is equal to 0.5 J/cm2 or less. When the energy density is equal to 0.03 J/cm2 or less, a contact angle of the liquid for temporary immobilization is equal to 20 degrees or larger, so that the improvement of the wettablility is insufficient. It is not preferred that the energy density exceeds 0.5 J/cm2, since damage caused to the resin layer is impermissibly large when the thickness of the polyimide system resin layer is equal to 0.5 xcexcm or less.
As light energy which satisfies the above conditions, an excimer-laser beam can be used in the present invention.
Irradiation by the excimer-laser beam is performed by throwing a pulse of 20 ns to 100 ns (pulse duration is 20 ns to 100 ns) at least once. The pulse duration is set to 20 ns to 100 ns, because the pulse duration of the excimer-laser beam is equal to from 20 ns to 100 ns at the present technique level, but it is not an absolute condition. The energy of each pulse of the irradiation pulse of the light energy exceeds 0.03 J/cm2 and is equal to 0.5 J/cm2 or less. At least one pulse of the light irradiation is necessary. Although the number of pulses can be one or more, the cost increases when the number is set to an unnecessarily large number, so that it is usually set to 5 times or fewer.
As a liquid for temporary immobilization, for example, an alcohol system liquid such as tetraethylene glycol or pentaethylene glycol or an ester system liquid such as ethyl salicylate can be used.
A proper amount of the liquid for temporary immobilization is adhered to a predetermined area by, for example, dropping the liquid in the predetermined area. An adhesion amount of the liquid for temporary immobilization is set to an amount sufficient to wet the predetermined area on the surface of the high polymer material to temporarily immobilize the electronic parts and ordinarily set to 4.5 cc to 13.5 cc for an area of 16 mmxc3x9716 mm (that is, 0.017 cc/mm2 to 0.053 cc/mm2).
According to the method of the present invention, by temporarily immobilizing the electronic parts on the high polymer material layer on the substrate, the electronic parts can be easily soldered at positions where the electronic parts are temporarily immobilized in a step of the fluxless reflow soldering after that.
In order to solder the electronic parts on the substrate, a metal pattern for the soldering can be provided in at least an area to be soldered on the substrate. The metal pattern in this case is constructed by a solderable metal (for example, Cu) and the thickness can be any thickness as long as it is easily soldered. The soldering can be performed in accordance with, for example, the micro Ball Grid Array (micro BGA) method or CCB (Controlled Collapse Bonding) method. A metal layer of a predetermined shape is provided on the surface of the base material of the substrate, and the high polymer material layer having holes corresponding to predetermined positions of the metal layer is coated on the substrate on which the metal layer is provided. On the other hand, a solder ball is provided for the electronic part at a position corresponding to the hole section and the solder ball and the metal layer exposed at the hole section of the high polymer layer are aligned, thereby temporarily immobilizing the electronic parts.
In the method of the present invention using the irradiation light of the energy density in the above range, the metal pattern for soldering is not damaged and the surface reformation of the high polymer material as an insulating material can be performed.